Field of the Invention
The invention is based on a method for fabricating a lithographic reflection mask, in particular for feature sizes below 100 nm. For the fabrication in particular of semiconductor chips on a silicon wafer (wafer), it is already known to use reflection masks that contain the structures that are projected onto the wafer. In order to optimize and reduce fabrication costs, the semiconductor industry endeavors to make the feature sizes on the semiconductor chips as small as possible. The miniaturization of the structures allows more transistor functions to be realized in an extremely small space.
Known optical lithography methods enable feature sizes of approximately 100 nm to be controlled in terms of production engineering. However, goals for the next few years are for the feature sizes to be considerably reduced; by way of example, line widths of 35-70 nm are sought for extremely small structures. Known optical lithography methods, in which wavelengths of 157 nm are used, for example, can no longer be employed for the small structures since a technologically and economically dictated limit is reached for the wavelength with a resolution of approximately 70 nm that can presumably be obtained.
In order to achieve smaller feature sizes, it has already been proposed to use a shorter-wave radiation in particular in the extreme ultraviolet region (EUV radiation). An extreme ultraviolet lithography method (EUVL method) has been disclosed in which use is made of a soft X-ray radiation in the range from 10 nm to 14 nm. Since no refractive materials (lenses) exist for the radiation, multilayer-coated reflection elements are used in the corresponding exposure systems (stepper, scanner) for the illuminator, the imaging optical configuration and the mask. The structures on the reflection mask are imaged with their size reduced on the wafer by EUV radiation which is incident obliquely on the mask and reflected there and a multimirror optical configuration.
Such an EUVL method is disclosed for example in the publication by John E. Bjorkholm, titled “EUV Lithography—The Successor to Optical Lithography?”, Intel Technology Journal, 3rd Quarter 1998. The publication proposes a four-mirror projection system which is used for ultraviolet radiation with a wavelength of 10-14 nm. For first experiments, a Schwarzschild mirror system with oblique illumination of the reflection mask was used in order to prevent shadowing of the EUV radiation by the mirrors. As a result, only small mirror partial areas outside the optical axis are utilized, resulting in an effective numerical aperture (NA) of approximately 0.07. In this case, the structures on the mask are imaged with their size reduced by the factor 10 on the wafer.
Furthermore, two fabrication methods for an EUVL mask have been disclosed, which are patterned using an absorber etching process or using the so-called “damascene” method. The two methods proceed from a mask substrate to which a multilayer layer is applied as a reflection layer. In the case of the first-mentioned method, a buffer layer is deposited onto the multilayer layer and an absorber layer is then deposited onto the buffer layer. By use of electron beam lithography using corresponding etching methods known per se, the structures are transferred to the absorber layer or the buffer layer.
In the case of the second-mentioned method, the multilayer layer is patterned by electron lithography and with the assistance of anisotropic etching methods. The resultant depressions in the multilayer layer are completely filled with absorber material by deposition of an absorber layer and subsequent polishing (“damascene technique”).
The patterned mask blanks produced according to the methods outlined above then later serve in both methods as imaging objects that are projected with their sizes reduced onto the wafer. Depending on the fabrication method used, the absorber layer deposited after the multilayer layer is completely inserted into the reflection layer or disposed above the latter.
What is disadvantageous, however, in the case of the two methods mentioned above is that shadowing effects of the structures lying above the reflection layer, which are caused by the absorber and buffer layers, cause or amplify disturbing imaging errors (structure width alterations and structure displacements).
Furthermore, the fact that asymmetrical intensity profiles of the reflected radiation are produced during oblique irradiation is unfavorable. The profiles are amplified by the fact that a part of the reflected radiation passes under the buffer and absorber layers and is absorbed there and can thus lead to asymmetrical resist structures on the wafer.
A further problem is seen in the fact that, as a result of different thermal expansion coefficients of the mask materials, temperature gradients arise both during the deposition of the materials and during operation, which generate mechanical stresses and distortions and which can likewise lead to structure alterations. Moreover, these stresses can also lead to structure alterations in the multilayer layer and at the same time alter the reflectivity of the multilayer layer.